1. Field of the Invention
This invention relates to an optical device for a diode array, and more particularly to an optical device for imaging the output of a diode array with a single optical element of unitary construction.
2. Description of Related Art
Conventional lasers have beams with circular symmetry. Standard optical lenses are frequently used to image their output beams into bright spots. Laser diodes form efficient pumping sources. Laser diodes have elliptical beam properties.
However, problems arise in focusing the output beams of diode arrays and bars onto a small spot or zone. In a diode bar, light energy from an active layer or region at a cleaved facet or edge of the diode in a slit-like pattern. This pattern has an oblong geometric configuration. Typical dimensions for a slit in a laser bar can be one micron high by 400 microns wide. The output beams emerge at large divergence angles from the small dimensions with a significantly lesser divergence angle from the large dimension.
With a single diode emitter the m2 in the slow axis is much greater than the m2 in the fast axis. The difference between the slow and fast axis is substantially increased for a diode array.
A pair of lenses has been used to image the output from a diode bar. In one embodiment, a lenticular array is used. The lenticular array changes the angle of divergence from a wide angle to a narrow angle for each diode laser. In this embodiment, there is one-to-one lens associate with each diode emitter. A second lens is used as an objective and focuses light to an image plane where a plurality of spots are formed. In this system, total convergence of the beams is not achieved.
Many techniques for collecting and subsequently focusing the output beam from a diode bar have been demonstrated for use in a variety of applications and in particular for end-pumping of solid-state lasers. Most of these techniques employ either an arrangement of cylindrical and spherical lenses or involve coupling light from each emitting region into a respective optical fiber.
In both of these cases, the minimum average beam diameter which can be produced and maintained over a length of a few millimeters is relatively large (typically greater than 1 mm). One example of a typical focus for a diode bar is 0.1 mmxc3x972 mm with an numerical aperture of about 0.5. This degree of focus can not be used to get into an optical fiber with a numerical aperture of about 0.2xcfx86 of about 0.5 mm.
Another disadvantage of these previously proposed techniques is the power loss produced by the arrangement of focusing optics or fibers. Again, this reduces the usefulness of the devices in applications such as end-pumping or surgical treatment devices where a high power output is required.
Fiber converters have been used for imaging the output of diode bars. See Grag, et al., Opt. Lett. 18, 1317 (1993). Image slicers have also been used to image the output of diode arrays. Image slicers divide a stellar image into parallel strips. This radiation is then redirected to form a line image with the strips running end to end. The resultant image is much narrowed compared with the original circular image and is well matched to a slit shape. Greater efficiency of the stellar radiation is achieved for a narrow slit.
For diode bars, it is desirable to progress from a line source to a symmetrical area source. This is essentially the operation of an image slicer in reverse. Several image slicer systems have been developed.
A stripe stacker has been used for diode bars. See Edwin Optics Letters 20(2), 222-224 (1994). The stripe stacker was used to operate an image slicer in reverse. The highly divergent diode output was initially collimated by a rod lens made of fused silica. The rod lens had a length of 122 mm and a diameter of 1.6 mm. A collimated beam with a cross section of approximately 1 mmxc3x979 mm was created. The image was reformatted into a symmetrical area image by dividing the line source into three slices with a stripe stacker.
The stacker included three plane mirrors of precise thickness and shape. The plane of each mirror was inclined at an angle of 45xc2x0 to the plane containing the diode bar output facet. Collimated beams were rotated by 90xc2x0 at the mirror reflections. Each mirror intercepted the output from four diode stripes to create a 4xc3x973 array of spots in a diamond shape. The stacker approach suffers a number of disadvantages including loss of output power and polarization.
In U.S. Pat. No. 5,825,551, the output from a laser diode array is collected using a collimating lens. The beam is then imaged by a combination of orthogonally arranged cylindrical and spherical lenses. Pump radiation enters a multiple reflection beam shaper which includes two parallel plane mirrors. If there are n emitters, the Nth emitter is reflected 2N times to place it above the first emitter. The (Nxe2x88x921) emitter undergoes 2(Nxe2x88x921) reflections so there is a large path difference between beams. This causes a difference in size of each strip in the stack, giving a greater divergence, as well as the loss at each mirror is increased. This beam shaping device reconfigures the transverse spatial intensity profile of the beam such that the number of times by which the beam""s divergence exceeds the diffraction limit in one plane, (i.e., the m2 value for that plane, can be reduced) it is increased in the orthogonal plane.
There is a need for a simple optical system for imaging the output of a diode. A further need exists for a system that images the output of a diode with a single reflection where alignment sensitivity and losses are reduced. Yet a further need exists for a single optical element to image the output from a diode array. Still a further need exists for a simple optical system for imaging the output of a diode bar that provides easy alignment, preserves polarization and uses a single optical element of unitary construction.
Accordingly, an object of the invention is to provide an optical system for imaging the output of a diode array.
Another object of the invention is to provide a single optical element, optical system to image the output of a diode bar.
Still another object of the invention is to provide an optical system with minimal alignment difficulties for imaging the output of a diode bar.
Yet another object of the invention is to provide an optical system for imaging the output of a diode bar that preserves polarization.
A further object of the invention is to provide an optical system to image the output of a diode bar that uses a single optical element that is of a unitary construction.
These and other objects of the invention are achieved in an optical device with a mount, mount surface and a mount longitudinal axis. A single emitter diode source produces a first beam with an m2 in the horizontal direction. A prism array is included with a plurality of reflective prism faces. The prism array is positioned relative to the single emitter diode source to redirect the first beam and form a reformatted beam with an improved m2 in the horizontal direction.
In another embodiment, diode source has a first diode emitter producing a first beam, a second diode emitter producing a second beam and a third diode emitter producing a third beam. The diode source is mounted on the mount surface at an angle xcex8 relative to the longitudinal axis of the mount. A prism array has a first prism face, a second prism face and a third prism face. The prism array has an angled surface cut at the angle xcex8 relative to a longitudinal axis of the prism array. The first, second and third beams are incident on the first, second and third prism faces respectively. Optical path lengths between the first, second and third diode emitters relative to the first, second and third prism faces are equal.
In another embodiment, a diode source has a plurality of diode emitters that produce a plurality of diode output beams. The diode source is mounted on the mount surface at an angle xcex8 relative to the longitudinal axis of the mount. The diode output beams collectively form an overlapped line source at a distance from the diode source equal to x. The prism array has an angled surface cut at the angle xcex8 relative to a longitudinal axis of the prism array and is mounted above the mount surface at a distance from the diode greater than x. The prism array includes a plurality of prism faces. Each of the prism faces in the array reflects approximately equal fractions of tie beam.
In yet another embodiment, the present invention is a laser system. A high reflector and an output coupler define a laser cavity. A gain medium is positioned in the laser cavity. A mount has a mount surface with a longitudinal axis. A diode source includes a first diode emitter producing a first beam, a second diode emitter producing a second beam and a third diode emitter producing a third beam. The diode source is mounted on the mount surface at an angle xcex8 relative to the longitudinal axis of the mount. The first, second and third beams collectively define a pump beam. A prism array includes a first prism face, a second prism face and a third prism face. The prism array has an angled surface cut at the angle xcex8 relative to a longitudinal axis of the prism array. The angled surface of the prism array is positioned just above the mount surface. The first, second and third beams are incident on the first, second and third prism faces respectively. The optical path lengths between the first, second and third diode emitters relative to the first, second and third prism faces are equal. An intensity profile of the pump beam is tailored to produce a parabolic temperature gradient across the gain medium.